1. Technical Field
The present invention relates to a method of modifying qualities of materials such as metals and resins by a weld cladding method. The present invention also relates to welding wires used for the weld cladding method and a method of manufacturing such wires.
2. Background Art
Methods that can improve characteristics of a material by modifying its qualities at local parts thereof are in increasing demand in recent years. The automotive parts industry is focusing on, for example, thermal and mechanical loads against a cylinder head which increase as the engine output is raised. This raises a serious problem since the cylinder head is made from cast iron. Specifically, as shown in FIG. 26 of the accompanying drawings, the thermal cracks are apt to be generated in a bottom face 2 of the cylinder head 1, particularly at those parts which are thinner than other parts such as a part extending between an intake port 3 and an exhaust port 4 (called "inter valve part") and a part 6 surrounded by the ports 3 and 4 and a fuel injection nozzle opening (or a preheating chamber) 5. Therefore, the material of the cylinder head 1 should be strengthened at these local parts. In the case of a piston 7, as another example, parts surrounding a combustion chamber 8, particularly parts indicated by hatching in FIG. 27, need a strength-improvement treatment.
The following surface modification methods have been proposed to improve the strength of a local part of a material:
(1) Alloying: To improve a thermal resistance by adding an appropriate amount of heat-resisting alloying elements (e.g. Cr, Mo, Cu and/or Ni);
(2) Cast cladding: To form a cladding of steel or heat-resisting metallic material over the part liable to thermal cracking by means of casting; and
(3) Nitriding: To form a nitride layer on the surface either in gas or liquid after machining.
If the part in question is of an aluminum alloy casting, the following methods are avilable in addition to (1) and (2):
(4) Precipitation toughening: To raise tensile strength as well as ductility by the precipitation toughening effect arising on addition of Sr or Ti in an appropriate amount;
(5) Hard alumite (or anodizing) treatment: To improve the thermal resistivity by forming a hard alumite film of about 100 micrometers on the surface;
(6) Remelting grain refinement: To improve the thermal resistivity by letting the aluminum matrix undergo a rapid melting-and-solidification with a high density energy such as laser, electron beam and TIG so as to refine the granular structure; and
(7) Remelt alloying: To add heat-resisting elements (e.g. Ni, Cu, Cr and/or Mo) to the part melted in (6).
These methods, however, have following problems:
(1) Alloying: In addition to an obvious rise in cost, the castability is degraded on addition of the alloying element, which results in casting defects such as shrinkage, and aggravation of productivity and yield. Further, it is often the case that the alloying does not improve the thermal resistivity but deteriorates the thermal resistivity because of the generation of microshrinkage;
(2) Casting cladding: Because of the added care needed for oxidation prevention and casting temperature control during the casting, and the nondestructive testing afterwards, all of which are absolutely necessary to ensure satisfactory cladding, the increase in cost is enormous; and
(3) Nitriding: In addition to the obvious increase in cost, the high temperature of nitriding (560.degree. to 580.degree. C.), to which the machine part should be exposed entirely, often gives rise to deformation of internal cracking. Further, in order to perform the nitriding to perfection, the graphite in the cast iron should be removed.
Another proposal is to spray coat the part concerned here (the bottom face of the cylinder head) with a ceramic or a heat-resisting material. Since the spray coated layers are so liable to early separation, however, this method has not been implemented.
For the methods relating to aluminum castings, on the other hand, there are the following drawbacks:
(4) Precipitation toughening: Although this approach is simple enough to practice the benefit is small;
(5) Hard alumite treatment: Although the effect of improving the heat resistivity is greater than the methods (1) and (4), an application of alumite film to only the concerned part is costly because all other parts must then be protected from being converted into alumite;
(6) Remelting grain refinement: Though the operation is simple and the effect is comparatively large, the improvement obtainable is not large enough for the target demanded today;
(7) Remelt alloying: Though this method is basically more promising than the method (6), the formation of a sound (i. e., free of defects and of post-treatment crackings) remelting-alloyed layer is difficult. In addition, the content of alloying element must be held to less than 10 to 20% to prevent alloying element must be held to less than 10 to 20% to prevent these defects. Therefore, a remarkable improvement cannot be expected in the heat resistivity.
Moreover, there is another aspect of automotive parts that should be taken into consideration: For weight reduction, polymers (i.e., resins) are used more and more not only for coverings, inner panels and the like but also for outer panels and even for structural members.
Now, to overcome the deficiencies inherent with the conventional methods (1) to (3), there has been proposed a method to apply the weld cladding of heat-resisting metal (e. g., Co, Ni and Mo) to the parts that are liable to thermal cracking such as the bottom face of the cylinder head. The problem here has been the precipitation of cementite (Fe.sub.3 C) at the boundary between the base metal (cast iron) and the cladding heat-resisting metal, because the cementite is not only brittle but aggravates machinability. Also, the cladding of heat-resisting metal itself is liable to cracking during weld cladding.
A method similar to the above-described weld cladding has also been proposed for aluminum alloy castings to avoid the deficiencies of the conventional methods (1), (2) and (4) to (7). Here, because of the liability of in- and post-weld cladding, cracking, as in the method (7), the content of added alloying element cannot be raised sufficiently and the desired heat resistivity will not be obtained.
As for the resins, there are many problems, too. For example, when a resin is used for a structure member of an automobile, what is needed beside the strength is wear resistivity. Although the resins generally possess a high wear resistivity against contact with metals, they are apt to undergo plastic deformation if the contact load between the resin and the metal is large. This causes seizing and wear. To improve the rigidity and wear resistance of such a resin at its contact plane, a method of weld cladding it with a hard plastic using ultrasonic has been proposed. However, mere cladding of resin with hard plastic does not improve the characteristics. In addition, an increase in the elastic modulus, which is the most desired factor in preventing the plastic deformation under a heavy load, is scant. Therefore, this method hardly contributes to improvement of the wear resistance of the resin concerned.
As another example, the resins are used in the automobiles as housings of various onboard sensors and electronic controllers. In these applications, electrically conductive plastics are used to protect the components from electromagnetic waves and from trouble relating to electric waves.
An electronic instrument is often desired to be attached to an existing instrument. In this case, the former instrument may be mounted on a housing of the latter instrument, and a vibration analysis must be conducted since the vibrations affect the electronic instrument, but antivibration measures for the combined instruments require a great amount of expense and labor even if adequate vibration-damping measures are given to the individual instruments. A simple way of solving this problem is to place a rubber material between the instruments. But, since such the material does not accept paint or reflects differently as compared with the instruments next thereto, the appearance of the product is not pleasant and the value as a merchandise is lowered.